1. Field of the Invention
This invention pertains generally to near-field optical devices and methods for reading and writing on optical media. More particularly, the invention is a single mode laser apparatus and method for near field optical information reading and writing, wherein both reading and writing modes are achieved in a single mode laser, and where laser voltage variation, as well as rear facet power, may be utilized as an output signal.
2. Description of the Background Art
Optical information storage technologies have provided increasing storage densities over the years. Conventional far-field techniques for reading and writing optical media utilize a collimated laser beam which is focused onto the optical medium by an objective lens. For a laser beam of wavelength xcex and an objective lens with a numerical aperture NA, a read/write spot size of xcex/2NA is obtained. These techniques have allowed reading and writing of optical media having storage densities in the Gb/in2 range.
Far-field read/write techniques are generally limited by the light wavelength and numerical aperture of the focusing optics used for reading and writing. Increasing the NA to greater than approximately 0.6 results in rapid increases in astigmatisms and beam abberations. Closer positioning of the objective lens and medium can overcome these problems to a certain extent, but results generally in decreased reliability. Presently, commercially available storage systems having output wavelengths of about 650 nm provide storage densities of about 2-3 Gb/in2.
Use of shorter wavelength semiconductor lasers will allow increased storage densities. InGaAlP double heterostructure lasers have provided outputs in the 600 nanometer range. Edge emitting and surface emitting devices based on ZincBlende Group III-V materials such as ZnSe, CdZnSe and MgZnSSe are known which operate with blue-green output. More recently, edge and surface emitting lasers based on AlGaN and AlGaInN have been shown which provide outputs approaching the ultraviolet. These shorter wavelength laser devices, however, tend to have limited output powers, limited operational temperature ranges, and are subject to materials limitations which have so far resulted in poor reliability and relatively rapid deterioration. Further, the far-field diffraction limitations noted above will probably limit increases in storage densities to around a factor of two to four, even if the most promising short wavelength semiconductor lasers become commercially viable.
One approach to increased storage densities has been through the use of a solid immersion lens (SIL) positioned between the objective and the optical medium, which has allowed an increase in NA which is proportional to the refractive index of the SIL material. This technology has resulted in a spot size (diameter) of around 600 nanometers, and storage densities approaching 10 Gb/in2. Refractive index limitation of SIL materials, however, will prevent further increases in storage densities using this technology.
A more promising approach to increased optical storage densities has been through development of other near-field optical data storage techniques, which offer potentially high data storage densities and readout rates. These near-field optical techniques require the use of radiation source apertures and distances on the order of generally less than the wavelength xcex of the radiation source to allow high storage densities. With aperture-based near-field techniques, storage densities are limited primarily by the light aperture size and distance between the aperture and optical medium. Using currently available semiconductor lasers and near-field techniques, storage densities on the order of around 100 Gb/in2 appear possible.
Small aperture light sources for near-field applications have been obtained through tapered optical fibers with metallized sides, which have provided apertures as small as 200 nanometers. Tapered fiber devices, however, have only been able to provide an output power of around 50 nW for input power into the fiber of 10 mW, with higher input power resulting in catastrophic failure at the light emitting aperture. This low output power severely limits data rates for optical storage using currently available media.
A more attractive near-field technology has involved use of small features associated with the emission facet of semiconductor laser devices, such that a substantial portion of output power is directed through a small area of the emission facet. Surface emitting and edge emitting devices have been made which have an emission facet aperture, of xcex/2 width or smaller, through which one half or more of the total emission can be directed. Edge emitting devices have also been made having tapered slits, normal to the emission facet, which are filled with low refractive index material to define waveguides which sharpen the beam spot from the emission facet.
The known near-field semiconductor laser structures exhibit an number of features, particularly with regard to output power and readback signal. When the internal mode of a semiconductor laser is coupled to the external mode between the reflective surface of the optical medium and the laser output facet, as occurs in near-field operation in reflection, the laser threshold current and laser output intensity will vary according to feedback light reflected back into the laser from the optical medium. During readout, low reflectivity spots on the medium providing less reflective feedback, and high reflectivity regions providing correspondingly more reflective feedback. For a wide area output aperture, it has been previously proposed that these variations be designed such that the laser undergoes transitions above and below laser threshold. However, the resulting changes in laser threshold and output power can limit data rates due to turn-on delays and relaxation oscillations in the laser. Further, during the writing and erasing processes, the reflectivity fluctuations result in unstable, fluctuating output power from the laser front facet, limiting the control of data writing. On the other hand, a substantial change in output power at the laser rear facet facilitates use of rear facet output for readback. Previously available semiconductor lasers for near-field use have been unable to provide stable, front facet output power for effective writing, together with fluctuating rear facet output power for effective readout.
There is accordingly a need for a near-field laser and detector apparatus and method which can operate at high data rate, which provides stable, high output power from a front facet for effective writing, and which provides substantial modulation in output power from a rear facet for effective readout. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in the background art.
The invention is a near-field laser and detector apparatus and method wherein both writing and reading optical media can be carried out using the same or very similar small-apertured lasers. The reading and writing modes provided by the invention can be accomplished with a single mode laser, without multimode operation. The single operational mode can be utilized with both edge emitting and surface emitting laser configurations, and allows readout via rear facet output power variation or laser voltage variation.
In general terms, the invention comprises a small aperture laser operating with a bias current higher than a threshold current associated with feedback from a high reflectivity portion of an optical medium, and a voltage detection system associated with the p- and n- contacts of the laser. An optical detection system may alternatively be used instead of the voltage detection system.
By way of example, and not necessarily of limitation, the laser comprises an active region positioned between a first conductivity-type clad region and a second conductivity-type clad region. The active region, first conductivity-type clad region and second conductivity-type clad region define generally a laser cavity. A first, front emission facet on the laser includes an aperture thereon, with the aperture having a width which is generally smaller than the output wavelength of the laser. The first emission facet is positioned proximate to an optical medium during operation.
The active region, first conductivity-type clad region and second conductivity-type clad region are structured and configured such that the laser operates with a bias current which is greater than the threshold currents associated with feedback from both high and low reflectivity regions of an optical medium. More preferably, the bias current is such that output power fluctuation from the front facet of the laser is minimal, while output power fluctuation from a rear facet of the laser is substantial.
A first electrical contact is associated with the first conductivity-type clad region, and a second electrical contact is associated with the second conductivity-type clad region. A voltage detection circuit is placed in electrical communication with the first and second electrical contacts via first and second electrical conductors. Voltage fluctuations associated with feedback from the high and low reflectivity portions of the optical medium are detected by the voltage detector circuit.
In a first embodiment, the laser is an edge emitting semiconductor laser wherein the first emission facet is generally normal to the plane defined by the laser active region. A second, back emission facet, which is substantially parallel to the first emission facet, faces away from the optical medium. An optical detector may be included proximate to the second emission facet and positioned to read the optical output from the second emission facet. In a second embodiment, the laser is a vertical cavity surface emitting semiconductor laser.
The laser and detector of the invention are associated with an optical head having an aerodynamic slider which is coupled to a read/write arm via a suspension system. During operation, the optical head preferably is positioned adjacent an optical medium, which may comprise a phase change material such as a GeTeSb alloy, to allow multiple write and erasure events. Preferably, the laser is positioned such that the internal mode of the laser is substantially coupled to the external mode between the reflective surface of the optical medium and the first emission facet.
In one embodiment, during writing the laser is pulsed to locally heat the high reflectivity polycrystalline phase change material above its melting point to create an amorphous melt spot of relatively low reflectivity, which is of comparable size to the aperture area on the first emission face. Readout is obtained by flying the laser over the optical medium and measuring the changes in reflection associated with the low reflectivity amorphous melt spots and higher reflectivity unmelted, polycrystalline regions of the medium.
The laser threshold current and laser output intensity vary generally according to feedback light reflected back into the laser from the optical medium, with the amorphous spots on the medium providing less reflective feedback than the high reflectivity polycrystalline regions. During both writing and readout, the bias current for the laser is maintained at a level higher than the laser threshold current, such that minimal variation in output intensity occurs from the first, front emission facet, while substantial variation in output intensity is achieved from the second, rear emission facet. Readout may be effected by optical detection of output from a rear facet, or by measuring voltage variation associated with the electrical contacts on the laser, which are positioned to monitor voltage modulation associated with output power fluctuation from the rear facet. Operation in this manner allows both reading and writing with the same laser operating in a single mode.
Further advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing the preferred embodiment of the invention without placing limitations thereon.